In the available art, the non-contact sensing and pneumatic gauging is based on measuring the backpressure of compressed air flow to a nozzle in very close proximity to a workpiece surface. In a prior art conventional pneumatic gauging circuit, the air gap distance between the nozzle outlet and the workpiece surface is estimated by using the back pressure in a chamber through which compressed air is flowing via a restricted orifice. Such back pressure is function of the distance that separates the tip of the nozzle connected to the chamber and the adjacent surface of the workpiece. The supply air pressure to the back pressure circuits is precisely regulated.
The back pressure measurement circuits are very sensitive to regulator pressure variations because the output pressure variations appear as common mode noise to the detector. To reduce this sensitivity, a back pressure measurement information can be taken in a differential mode. In this mode, two air flow branches are used from a common inlet to separate outlets. The flow of air to a measurement pressure chamber is regulated in each branch by a fixed control orifice. One of the pressure chambers is connected to a pneumatic gauge nozzle affected by the measurement air gap. The other pressure chamber is vented to atmosphere through a reference orifice for datum control. An expansible bellows is connected in prior art systems to each pressure chamber. A flexible diaphragm can also be used to separate the two pressure chambers. The differential motion is detected and measured as a function of the difference of pressure in the pressure chambers. The extent of such differential motion is related to the air gap between the air gauge nozzle and the adjacent surface of the workpiece. Any fluctuations in air pressure due to poor regulation, or due to the temperature fluctuations, will cancel if back pressure is the same on both sides.
The measured air back pressure can be displayed relative to a calibrated scale graduated in thousandths of an inch or millimeters. In some prior art conventional pneumatic gauging systems, the measurement display instrument is electrically driven, even though it may be in the apparently traditional form of a vertical scale instrument. A prior art displacement transducer takes form of a variable transformer having a movable core for varying the mutual inductance between the primary and secondary windings of the transformer as function of the displacement of the core. The input member of the displacement transducer displaces the movable core. The primary winding of the transducer transformer is connected across a power supply. The output signal as a function of the linear displacement of the transformer core, can be read as an analog output of a vertical scale instrument imitating a U-tube manometer display, or as a digital output of a digital manometer display.
The U.S. Pat. No. 5,789,661, entitled "Extended range and ultra precision non contact dimensional gauge", and issued to Fauque in 1998, is incorporated in its entirety in the present patent application. The '661 patent discloses a non-contact pneumatic-electric wafer measurement system with accuracies better than 0.5 micron. A measurement head is held aloft over the wafer and base, and the tip of an air nozzle in the measurement head is directed at the wafer and automatically extended to near contact. The nozzle is servo-positioned by an air sensor and motor combination with an overall precision of positioning of about 3-4 microns. A linear displacement gauge is attached to the air nozzle and is used to determine the nozzle position to within 0.5 micron. The motor positioning error is removed by combining the linear displacement gauge reading with an estimate of the air gap derived from a reading of the air nozzle backpressure that has an accuracy of about 0.1 micron. Thus, the thickness of the wafer is determined with an accuracy of about 0.5 micron.
However, the system of '661 patent cannot be used for measurements of geometrical parameters (for example, the warpage and bow) of ultra-thin wafers with the thickness less than 200 microns. This is due to the fact that the ultra thin wafer with the thickness less than 200 microns vibrates under the air pressure that is used in the back pressure sensor of '661 patent, thus making it impossible to measure the curvature of an ultra-thin wafer.
What is needed is to extend the usage of the measurement system of '661 patent in order to measure the geometrical parameters of an ultra-thin wafer with the thickness less than 200 microns.